In such circuit arrangements, the transmitter and the receiver are interconnected via at least one signal line via which it is possible to exchange an item of useful information. During the transmission of the useful information, a current or radiation profile typical of the respective data is produced. If success is achieved in evaluating this current and/or radiation profile, it is possible to draw conclusions on the transmitted data and/or the functional set up of the circuit arrangement. Consequently, in designing circuit arrangements a high value is placed on avoiding data-dependent switching processes in the electronic circuit, which comprises, inter alia, the transmitter and the receiver.
Two different methods, inter alia, are known which can be used to analyze current profiles.
The differential current profile analysis (differential power analysis, dpa) is one of the most important methods for tackling circuit arrangements with regard to confidential information such as passwords or cryptographic keys. In this case, in conjunction with a prescribed program and/or a prescribed algorithm statistical methods are used to evaluate measured current profiles and/or their charge integrals of the circuit arrangement which are calculated over one or more clock cycles. Given a sufficiently large number of program runs, a conclusion can be drawn on the information to be protected from the correlation of a systematic data variation and the respective charge integral.
In order to complicate spying on the data, circuit arrangements are therefore executed using so called dual-rail technology. By contrast with conventional circuit arrangements using so called single-rail logic, in the case of which each bit within a data and signal path is represented physically by exactly one electric node, each bit is represented by two nodes k and kq in the implementation using dual-rail logic. The bit then has a valid logic value when k corresponds to the true logic value b of this bit and kq corresponds to the negated value bn=not (b).
The desired invariance of the charge integrals is achieved by inserting between in each case two states with valid logic values (b, bn)=(1,0) or (0,1) a so-called precharge state for which both the node k and the node kq are charged to the same electric potential. The nodes or signal lines consequently assume logically invalid values (1,1) or (0,0).
It holds for each such arbitrary state sequences that at each transmission from a precharge state to a logically valid value exactly one node is recharged from “1” to “0”, and at each transition from a logically favorable value to a precharge state exactly one node is recharged from “0” to “1”. This is independent of the logically valid value b of a respective flag bit.
It follows from this that the charge integrals corresponding to these state sequences are independent of the sequence of the logically valid values (b, bn), provided it is ensured that the nodes k and kq have the same electrical capacitances. The current profile of a data path implemented in such a way is therefore not a function of the time variation in the data to be processed. Differential power analysis cannot be used to spy on data in a circuit arrangement having dual-rail logic.
However, a disadvantage of the dual rail logic is an increased area requirement, a high current and a lower performance owing to the precharging phase required between two state sequences. On the other hand, it is possible to draw a conclusion on the transmitted data from the radiation profile by recharging the signal lines.
Another method for preventing current profile analysis is the so called switched capacitor method. In the case of this method, each signal line is connected to a charge store which is connected to the signal line, as the case may be, via a circuit arrangement evaluating the useful information. Although it is possible thereby to smooth out the current profile, it is, however, still possible as before to analyze the useful information and, if appropriate, to draw a conclusion on the mode of operation of the circuit arrangement by means of a radiation analysis.